1. Field of the Invention
The present invention relates to a memory device for writing data (information) by changing electrical characteristics of an interelectrode material layer between two electrodes.
2. Description of the Related Art
Conventionally, as a memory element capable of being easily formed in a simple structure, a microelectronic programmable element has been known (Japanese Translations of PCT International Application Publication No. 2002-536840). In the element, data is written by applying a given voltage to ionize silver (Ag) contained in an interelectrode material layer between two electrodes, moving the ionized silver to form a conduction path for electrons and the like, and thereby changing electrical resistance between the foregoing electrodes.
In the element, as a matter of convenience, a state that resistance between the two electrodes is high before applying a voltage is a storage state of data “0,” and a state that resistance between the two electrodes is low as a result that metal ions are diffused in the direction of the opposite electrode due to application of voltage is a storage state of data “1.” Further, an operation to change the element from the high resistance state to the low resistance state is called writing operation, and an operation to return the element from the low resistance state to the high resistance state is called erasing operation.
In the conventional memory element, it is disclosed that a resistance value of the conduction path, which comes into the low resistance state after writing is about 200 Ω, and a resistance value of the interelectrode material layer, which comes into the high resistance state after erasing is 1 M (mega) Ω or more. Further, it is described that a resistance value between the low resistance state of about 200 Ω and the high resistance state of 1 M (mega) Ω or more can be arbitrarily created by arbitrarily limiting a current value applied to the memory element in writing.
However, it is not allowed that the resistance value created by writing can be unconditionally large as long as, for example, a resistance change rate due to writing is sufficient.
That is, in the conventional memory element, it is necessary that when a writing voltage is applied, a voltage (threshold voltage) at which the resistance change is generated is 0.2 V, and when the written data is read, a voltage to be applied (reading voltage) is less than 0.2 V in order not to generate unintended resistance change for the memory element. Therefore, for example, when a memory element, in which a state of a high resistance value (storage state of data “0”) is 10 MΩ and a state of a low resistance value (storage state of data “1”) is 1 MΩ is constructed, even if a voltage of 0.2 V at the maximum is applied, flowing currents are only 0.02 μA and 0.2 μA, respectively. When these currents are flowed to load resistance of, for example, 1 KΩ to convert to a voltage, a voltage difference between the two is only 0.2 mV. However, in the related art, it is not easy to read the written data by detecting such a small voltage difference.